1. Field of the Invention
The present invention relates to an input/output device. It also relates to a method of operating such a device, and to a control system incorporating such a device.
2. Summary of the Prior Art
It is well known to control an object, such as an industrial system or plant, using a programmable controller. That programmable controller has a processing module and one or more input/output devices, which input/output devices link the processing module to the object to be controlled. Data from the object are transmitted via one or more of the input/output devices to the processing module, and the processing module may also transmit data to the object to be controlled via one or more other input/output devices.
An example of such a known controller is the H-2000 Programmable Controller from Hitachi, Ltd.
When originally developed, such input/output devices were controlled entirely from the processing module of the programmable controller. However, it has been proposed that at least some of the control of the input/output device be achieved via a processing means within the input/output device itself. Such an input/output device is then called an "intelligent" input/output device. The processor of an intelligent input/output device is arranged to carry out at least some processing functions on the data that the input/output device receives.
In existing arrangements, the data from an object is passed to the input/output device in a series of frames, with each frame corresponding to a string of data, and the string of data must then be processed by the processor of the input/output device. One example of the type of frame which can be used is disclosed in e.g. the article entitled The Hierarchy of Communication Networks in the Programmable Assembly Cell: and Experimental Framework" by L. Sintonen and T. Virvalo in IEEE Network of May 1988, Volume 2, No. 3, pages 48 to 52.